Activation of tumor suppressor p53 as a targeted nongenotoxic cancer therapy has been pursued enthusiastically for many years, because p53 possesses potent tumor suppressing activity in vivo. p53 can terminate cancer cell growth by induction of apoptosis and senescence and inhibit cancer cell growth by cell cycle arrest. The p53-based therapy is particularly attractive for cancer types including retinoblastoma, neuroblastoma and leukemia/lymphoma in which p53 is rarely mutated and p53-dependent apoptosis pathway is a predominant endpoint in these cancer cells. Except for cancer-selected p53 mutations, the tumor suppressor activity of p53 is mainly inhibited by p53-binding proteins Mdm2 and MdmX in normal and cancer cells. Prior focus of p53 reactivation strategy has been on targeting the Mdm2-p53 interface, and/or MdmX-p53 interface. Exploration of these inhibitors led to discovery of a list of potent Mdm2 inhibitors, several compounds of this class have been advanced to Phase I clinical trials in hematological neoplasia and solid tumors. However, the therapeutic effects of Mdm2 inhibitors can be attenuated by MdmX overexpression. Although peptide inhibitors of dual function for inhibiting both Mdm2-p53 and MdmX-p53 interaction will overcome this problem and enhance p53-dependent cancer killing, these inhibitors will not inhibit Mdm2 E3 ligase activity toward non-p53 targets such as RB, p21 and DAXX, which to different extent affects the p53-dependent biological effects.
The TP53 pathway is the most frequently inactivated pathway in human cancer. Overexpression of Mdm2 and MdmX (aka Mdm4) is the major mechanism for inactivation of p53 protein function in hematological malignancies. Targeting Mdm2-p53 interface has been a focus for development of anti-cancer drugs for p53 restoration in cancer therapy. However, MdmX overexpression confers resistance to this type of inhibitors such as Nutlin3a.
Recent genetic studies indicated that RING domains of Mdm2 and MdmX are required for in vivo inhibition of p53 activity during development. MdmX was reported to stimulate Mdm2-mediated p53 multiple monoubiquitination using GST-fusion Hdm2 proteins.